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Neutron methods

H/D Conrast Variation

The particular strength of neutron scattering in the study of soft-matter systems is based on the possibility of generating specific scattering contrast by using different isotopes of the same element. Soft-matter systems use almost exclusively the replacement of light hydrogen (H) by the heavy hydrogen isotope, deuterium (D). The scattering properties of these two isotopes are drastically different; a deuterated molecule in a protonated environment (or vice versa) is clearly visible.
In combination with appropriate chemistry, one can investigate otherwise unattainable structural and dynamic properties of individual components of complex systems.

An elementary classical example is the elucidation of the shape (conformation) of a long, flexible polymer molecule in the melt. Without variation in the contrast of some of the polymer molecules, the shape of the individual molecule remains virtually invisible, as it is indistinguishable whether one molecule segment belongs to the same or another molecule (all look the same). Now, when a molecule is marked by H / D replacement, one can track the shape lengthwise through the other "colour" of the associated segments. The scattering intensity corresponds to the conformation of the molecule under consideration. In this way, in the pioneering period of neutron scattering and soft-matter systems, this question was clearly clarified by neutron small-angle scattering (KWS or SANS) on a mixture of isotope-labelled and "normal" polymers: the shape of the polymer chain corresponds to that of a so-called Gaussian tangle (quasi like a 3D random walk).

In a further step, the analysis of the change in speed of the neutrons during the scattering (inelastic or quasi-elastic scattering) can be used to infer the mobility of this Gaussian coil or other structures which each contribute to the scattering intensity. For this one uses then preferably the neutron spin-echo spectroscopy (NSE).

Isotope-labelled structures occurring side by side in a sample (e.g., an aqueous solution) can be viewed separately if the so-called scattered-length density of the solution is adjusted by isotope mixing (here, H2O: D2O).

Neutron small angle scattering

Using the many options offered by H/D contrast variations in polymer systems or other soft matter samples, SANS (Small Angle Neutron Scattering) enables scientists to gain a detailed insight into molecular conformation and the asociated behaviour of molecular components. The small angle scattering instruments KWS-1, KWS-2 and KWS-3 at FRMII in Garching make it possible to analyse structures on a scale ranging between 1nm and 1000nm. Where possible, accompanying images can also be produced if necessary using transmission electron microscopy (TEM).

Neutron small angle scattering (SANS) as a technique is also an exceptionally important tool to study material in an external field, when combined with macroscopic characterization techniques, such as rheology, for instance. Within the framework of our research activities, we analyse the structural response of polymers on a shear and extension deformation in space and time as well as in in-situ experiments.

Branched polymers behave differently from linear chains. Time-dependent phenomena taking place at different length scales can now be decoupled and studied in static experiments, by freezing each respective state. Quenched melts from partially labelled branched co-polymers can thus be studied using the in-situ SANS method and coupled extensional rheology, to investigate structural relaxation on different hierarchical levels. The topological tube is here closely related to the relaxation of stress.

The temporal development of structural changes, aggregations or phase transitions, ranging from a few seconds up to a number of hours, can be used in kinetic SANS measurements, e.g. by using so-called “stopped-flow” techniques.

Neutron spin echo spectroscopy

The ideal complement to SANS to observe the temporal developement of structures under observation under equilibrium conditions is neutron spin echo spectroscopy. With the J-NSE instrument at FRMII, the NSE instrument at SNS in Oak Ridge, instruments at IN15 and also shortly WASP at ILL, a wide spectrum of measurement options are available to JCNS.

The result of a SANS measurement is analogue to a snapshot of the structure, data from an NSE measurement can be seen as being approximately analogue to images with multiple long exposure times. The time parameter is the adjustable Fourier time (typically between 0.005ns .. 500ns). The NSE signal delivers the correlations of the scattering structures which are still available after this time lag. The times involved are significantly shorter than those obtainable using a kinetic SANS measurement.

With a kinetic measurement, the reaction of a synchronous macroscopic “disturbance” is measured. However, NSE measures the correlation time of thermal fluctuations (Brownian motion). In this way, diffusion and relaxation processes can be observed on a molecular level in polymer melts, solutions, proteins and other soft matter systems on their typical length and time scale.

Neutron back scattering spectroscopy

The back scattering spectrometer enables access to fast dynamic processes on a structural length (0.1-1nm) below the SANS range (SPHERES at FRMII and partly at BASIS in Oak Ridge). Back scattering spectroscopy observes dynamic processes on a time scale between around 10ps and ns. However, unlike the NSE in terms of the range of energy distribution of the scattered neutrons, the resolution is typically in the area of µeV (SPHERES: 0.7µeV, BASIS: 4µeV). Here, mostly the dynamics of the self-correlation of protons in the sample are measured, in the form of “incoherent” scattering, which helps considerably with detecting the signal.

As well as proton diffusion (e.g. in proton conducting or hydrogen storage materials), this signal gives us information about the mobility of molecular groups to which protons belong, and also for example, about the segment mobility of polymers or amino acids or water in proteins. Back scattering bridges the gap between NSE and time-of-flight spectrometry (TOF).

Neutron reflectometry and SANS, NSE and grazing incidence

The depth profile of macroscopic layer structures is made visible using neutron reflectometry. In the area of soft matter research, H/D contrast variations are again an important tool to increase the information content of a measurement. Lateral structures become visible in the diffused scattering in addition to offspecular scattering. This is used with SANS and NSE with a grazing incidence (GISANS, GINSE).

A special new method to increase intensity while using GINSE involves the use of multi-layered substrates, so that the coating of a resonator, where the amplitude of the evanescent matter wave of the totally reflected neutron beam (where the layer to be studied is positioned), is increased by more than one order of magnitude.

Other neutron techniques

Listed above are the mainstays of soft matter research in the area of neutron scattering. In a number of cases, however, other diffraction methods or spectroscopies are also utilized. These include diffraction with polarization analysis at DNS (an instrument for diffusion scattering in Garching), which enables coherent and incoherent scattering to be separated out, and time-in-flight spectroscopy, which offers a connection to back scattering (e.g. the instrument TOFTOF in Garching, or IN5 at the ILL).

The BIODIFF instrument enables macromolecular crystallography to take place over a wide temperature range, and is of prime importance in determining the proton positions in protein crystals.

X-ray small angle scattering

In-house X-ray scattering methods enable a rapid characterization of samples to take place, as well as SANS analogue measurements as long as sufficient contrast is available. With proteins in solution, whose details are only recognizable using a relatively large scattering vector, X-ray scattering is advantageous, as the spin incoherent scattering of hydrogen cannot raise the surface. However, radiation damage could occur in the sample. In-house facilities from Anton Paar (SAXSPACE), GALAXI and NANOSTAR U: (Bruker AXS) (point geometry) are available:

Transmission electron microscopy (TEM)

At the MLZ in Garching, JCNS undertakes cryotransmission electron microscopy JEOL2200FS with extensive equipment for preparing samples (e.g. cryoplunge, ultramicrotome, freeze fracture, vaporisation). This is seen to be crucial in the support of small angle studies.

As SANS delivers representative and quantitative mean value measurements and usually produces angle averaged scattering curves, TEM delivers direct images of the object in the sample. However, these are individual realizations and may be altered during sample preparation.

The combination of both methods is a powerful tool that aids TEM in selecting the correct structure model, and a good fit of suitable model parameters at the SANS data can deliver quantitative values under actual sample conditions.

NMR Diffusometry

Pulsed field gradient NMR produces diffusion data (in this case from protons in a sample) in the 10-micrometer/millisecond range. The acquired diffusion constants are equal to those measured by NSE in the nanometer range (100-nanoseconds).

In our institute, a pulse field gradient NMR (PFG-NMR, Bruker Minispec mq20 System with 33mm Airgap Magnet) is used as a complementary method, to observe the mobility of polymers (proton diffusion). To measure slower diffusion processes, e.g. in complex, functional polymer-based systems, the high resolution Bruker Avance 600 MHz instrument is available, with the sample head for diffusion measurements Diff30. In this way, measurements of protons as well as deuterons are possible.

PFG-NMR is used for studies on a time scale from a few milliseconds to several hundred milliseconds and is thus capable of supplementing neutron spin echo experiments.

Rheology

Linear rheology measures the frequency dependency of complex elasticity modules G(w) and provides the necessary connection between the mechanical properties and the molecular structure, as defined by scattering experiments. The dynamic modules scan the relaxation time scale in polymer melts in a sensitive way. Properties such as individual relaxation processes or plateau modules can be successfully discussed within a tube model for topological interplay between chains. This appears as a more or less distinctive maxima in G”-loss modulus at frequency proportional to the inverse relaxation time.

Dynamic studies complete the typical relaxation experiments in the macroscopic time window. They allow us to distinguish non-networked polymer melts from rubber and demonstrate, for example, within the framework of elastic materials, an analogy with temporary coupling, i.e. the topology and permanent network points of a vulcanized ensemble of chains. If one assumes that using changes in temperature, the characteristic times could be to the same extent speeded up or slowed down, then it is possible by means of the time-temperature overlay principle to establish a so-called master curve. Behaviour predictions can thus be made in areas where experimental work is either not possible, or very difficult to perform.

This simple-thermo-rheological behaviour is present in most homogenous systems. Deviations from this are observed in more complex systems such as mixtures with a different monomer chemistry or additional processes which can be activated such as all types of intermolecular interactions (complex formations, hydrogen bridges and so on).

Our understanding of polymers on the coupling scale is supplemented by non-linear rheology experiments. A major disruption of the system leads to a microscopic length scale-dependent deformation and rheological properties which are favourable to polymer processing. Due to the fact that this area has still not been investigated microscopically and dominated by over-simplistic theories, techniques such as rheology and coupling with neutron experiments are very important. The available scattering vector ranges as well as the time window open up interesting and relevant length scale and relaxation times.

Our rheometric equipment includes instruments capable of performing various measurement methods and modes:

AR-G2 Voltage-controlled rheometer for samples with low viscosity (liquids) with an added device for in-situ small angle light scattering for characterizing structures

Combined Rheo-IR measurements

The combination of oscillatory measurements of complex modules in the frequency domain with an extra analytical spectroscopic technique delivers an additional complementary information source for understanding processes in polymer materials. FT-IR is ideally suited to this, as it can determine structural changes in materials, in for example, reaction processes or inherent mechanisms. This is based on vibrational spectroscopy, by stimulating molecule particles with infrared light.

Based on certain characteristic vibrations of molecular units that are responsible for the processes being studied or significant macroscopic characteristics set out, a molecular fingerprint is acquired, which – if at the same time a mechanical disturbance is applied – provides, for example, an explanation of the complex dynamics of so-called supra-molecular polymers or superior structures. Recently, we have acquired a MARS III (Thermo Haake Scientific), connected to a ATR FT-IR (Nicolet).

• Mars III: Voltage-controlled rheometer with an FT-IR connection

Dielectric spectroscopy

Further insights in the internal dynamics can be gained – as long as the molecular units have a dipole moment – by the frequency- and temperature-dependent measurement of a complex dielectric function. The relaxation dynamic of soft matter – such as polymer materials and glassy liquids – are characterized by an enormously wide frequency range. Dielectric or impedance spectroscopy covers this area. It is based on the interaction of an external electrical field with an electrical dipole moment of the sample. Due to the fluctuation-dissipation theorem, the measured susceptibility is linked to the fluctuations of the local polarization, originating from the dynamics on a molecular scale. Neutron scattering is more limited with regard to frequency range, but provides a spatial resolution which is not possible to acquire using dielectric spectroscopy, so that both methods together give a more complete picture.

We have two instruments, one offering a frequency range of up to 10 MHz and the other up to 1.8 GHz.

Dynamic light scattering

In addition to the neutron scattering experiments, a further modern light scattering laboratory is available to our scientists. The use of light scattering is recommended when more information in a separate Q- and t-area is needed. The existing experimental equipment makes it possible to undertake static as well as dynamic measurements in the same experiment.

Differential Scanning Calorimetry

Calorimetry (DSC) can be used to research temperature-dependent phase transitions in any type of material. As well as polymers, this includes biomolecules, e.g. chocolate, cosmetics, through to metals and alloys. DSC is used in our laboratories mainly to research polymer phase transitions, such as glass and melt transitions. We also are equipped with a differential calorimeter (DSC) Q2000 from TA Instruments, with a temperature range of von -180°C to 725°C. Using modulated DSC, it is possible to separate reversible and irreversible parts of the thermal flux.

Methods for polymer synthesis and -characterization

In our own polymer synthesis laboratory, we can produce custom-made model polymers, for example, linear homo- and block copolymers, rings, branched polymers, such as combs and standard stars, as well as polymer grafted nanoparticles with defined molecular weight, functionality and narrow molecular weight distribution. The method of choice here is living anionic polymerization. For neutron scattering experiments, we routinely synthesize deuterium-labelled polymers based on deuterated momomers.
To undertake polymer characterization, the following methods are available:

• Size exclusion-chromatography: to determine relative and absolute molecular weights, e.g. molecular weight distribution.
• Osmometer: to determine the number average of the molecular weight, Mn.
• Multiple angle light scattering instrument and differential refractometer: used to determine the weight average molecular weight, Mw, and the specific refraction index increment, dn/dc.
• FTIR and UV/VIS spectrometer: used for the chemical characterization of monomers and polymers.
• Tensiometer using the drop method: for measuring surface and interfacial tension.
• Density meter using an oscillating U-tube: for the precise measurment of density in polymers, polymer solutions and solvents, in particular where it is necessary to calculate the exact contrasts in neutron scattering.

Additional important analyses, such as Maldi-TOF and NMR spectroscopy, are carried out by the Central Department for Chemical Analysis at Forschungszentrum Jülich.

Methods used in protein characterization

For characterizing protein solutions, the following methods are available in our biological laboratory: